An envelope of that type is described in the Applicants' document FR-A-2 698 851. In that known envelope, the additional flap is integral with the sleeve, and it is connected to the sleeve proper via two parallel lines of heat sealing which extend along generator lines of the sleeve, said lines of heat sealing defining the length of said flap.
The above-mentioned document provides tear means such as perforations disposed in two lines that are parallel to the generator lines of the sleeve and close to the two ends of the flap. The tear means must enable the additional flap to be detached easily without spoiling the sleeve in the separation zone where the flap is detached from the sleeve. It is possible to provide for the use of adhesive means instead of tear means, however it turns out that use of such adhesive means is difficult in practice.
Furthermore, the envelope shown in the above-mentioned document is suitable only for removing the additional flap in full, and not for removing a portion only thereof.
The above-mentioned document is a good illustration of the state of the art concerning the use of an envelope made of heat-shrink plastics material and having an additional flap provided with an explanatory sleeve disposed in the pocket formed by the flap and the sleeve. Nevertheless, it may be desired to use an additional flap forming an extension of an envelope for wrapping articles in some other way, and this is particularly true when it is desired to organize a coupon system in which one or more coupons can be detached from the envelope, e.g. for use as vouchers for immediate repayment or as vouchers for obtaining a price reduction when purchasing some other goods.
The state of the art relating to such use is well illustrated by document WO 92/02421 which shows a card envelope that is extended by an additional flap which terminates in a pull tongue that serves to break spots of adhesive organized vertically up the free end edge of the flap, so that the entire flap can finally be detached with the help of a line of perforations provided at the other end of said flap. Nevertheless, that technique is suitable only for envelopes made of reinforced paper or of card, and its teaching cannot be used for envelopes made out of a film of heat-shrink plastics material. If it were envisaged to use the same envelope structure with a heat-shrink plastics material, then two difficulties would be encountered: firstly it would be necessary to organize appropriate heat sealing along the free edge of the additional flap (capable of withstanding tensile forces encountered during shrinking), and then the pull tongue would need to be very thick, firstly to enable the line of heat sealing to be broken, and secondly to be strong enough to withstand deformation when the envelope is passed through the oven for shrinking said envelope.
Reference can also be made to document U.S. Pat. No. 4,318,235 which describes a paper label whose wall has a cutout enabling a central rectangular panel to be detached. In addition to the fact that the cylindrical envelope formed by the label does not have a flap, the teaching of that document is unusable for making an envelope out of heat-shrink plastics material since the cutout in question would not withstand shrinking forces.
Finally, the state of the art is also illustrate by documents U.S. Pat. No. 1,686,354 and BE-A-506 116.